Structured polarizers are used in many applications such as imaging polarimetry or interferometry. Due to the higher resolution of the image sensors, there is a high demand to have structured polarizers with a high spatial resolution in terms of portions with different polarization direction. Furthermore there is a high demand that the optically active layer of such polarizers is thin and provides a high polarization contrast.
One approach to obtain a structured polarizer with desired characteristics is the application of a wire grid onto an optically transparent material. The maximum resolution is however limited with this approach. Furthermore, the manufacturing costs are rather high. Also, the surface is very sensitive to mechanical contact or impact.
Another approach to obtain a structured polarizer with desired characteristics is the embodiment of metallic nanoparticles into an optically transparent substrate, whereby the nanoparticles such as silver are introduced in vicinity of the surface of the substrate e.g. as spherical colloids. In order to obtain a spatially structured polarizer, the spherical particles are then treated e.g. by heating and mechanical stretching or laser irradiation. Said treatment elongates and aligns the particles along a common direction which then exhibits polarization characteristics.
U.S. Pat. No. 7,256,937 B2 discloses a spatially structured polarizing device in which a plurality of superimposed structured polarizing substrates is used, each of the substrates having one common polarization direction over the whole substrate, the polarization direction of the substrates being different to each other, wherein each of the substrates can be structured by removing an upper portion of the substrate e.g. by etching a portion in which the metal particles are embedded. However, there is a need to have a plurality of superimposed structured polarizing substrates in order to obtain a structured polarizer having portions with different polarization directions because each substrate only provides one polarization direction. This approach therefore results in limited spatial resolution of adjacent portions with different polarization direction as the substrates are stacked on each other. Furthermore, the manufacturing costs are rather high due to the necessity of many stacked substrates.
DE 100 64 456 B4 discloses a spatially structured polarizing device as shown in FIG. 1 in which a plurality of portions 11, 12, 13, 14 having a different polarization direction are arranged side by side to each other within one layer. The boundaries between adjoining polarizing regions 11, 12, 13, 14 are depicted by reference sign 15. In order to obtain portions with different polarization direction, a pulsed laser beam with different polarization direction is irradiating onto the different portions causing the embedded metal particles to elongate and align along the beams' polarization direction. However, due to the Gaussian beam intensity profile 41 as shown in FIG. 2, the resulting non-linear response 42 of deformation with respect to laser intensity, the degree of particle deformation 43 is non uniform, i.e. a central region of each portion 11, 12, 13, 14 exhibits a higher polarization contrast compared to a peripheral region. This leads to blurred transition zones 44 along the boundaries between adjoining polarizing regions resulting either in regions 45 of insufficient or incorrect polarization (FIG. 1A) or in regions 46 of undefined or overlapping polarization. This however, significantly limits the spatial resolution of the structured polarizing device.
It is therefore an object of the present invention to provide a structured polarizer and a method for manufacturing the same which overcome the mentioned deficiencies in the prior art.